1. Field of the Invention
The present invention relates to a surface acoustic wave apparatus having greatly improved transmission characteristics and a communication apparatus including such a surface acoustic wave apparatus.
2. Description of the Related Art
Mobile phone (communication device) technology has advanced remarkably in recent years. In order to achieve such technology, surface acoustic wave apparatuses used in the radio-frequency (RF) stages of the mobile phones must provide higher performance. Such a known surface acoustic wave apparatuses is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-30367, in which surface acoustic wave resonators are connected in series to surface acoustic wave filters, particularly to longitudinally-coupled-resonator surface acoustic wave filters.
FIG. 3 shows this structure. In FIG. 3, a surface acoustic wave resonator 102 is connected in series to a longitudinally-coupled-resonator surface acoustic wave filter 101 having three interdigital electrode transducers (IDTs). The anti-resonant frequency of the surface acoustic wave resonator 102 is set to a frequency that is greater than the pass band of the longitudinally-coupled-resonator surface acoustic wave filter 101 and the resonant frequency of the surface acoustic wave resonator 102 is set within the pass band.
In such a structure, since the anti-resonant frequency of the surface acoustic wave resonator 102 is set to a frequency that is greater than the pass band of the longitudinally-coupled-resonator surface acoustic wave filter 101, the attenuation increases in the vicinity of frequencies greater than the pass band. However, since the resonant frequency of the surface acoustic wave resonator 102 is set within the pass band, the insertion loss within the pass band is not substantially increased.
Heretofore, characteristic selection has been performed in a wafer form, in addition to upon completion of a product, in the production of a surface acoustic wave apparatus in order to eliminate defective products produced in the manufacturing process. The elimination of defective products in a wafer form prevents the elements in the defective products from being mounted in packages, thus reducing package waste and achieving cost reduction.
Dedicated characteristic selection pads are provided for performing characteristic selection in the wafer form through a measurement using a probe. These characteristic selection pads are usually connected adjacent to electrode pads for connecting the surface acoustic wave resonator to packages for the purpose of reducing the capacitance in signal lines. The dedicated characteristic selection pads are provided because the electrode pads can be damaged by contact with the tip of the probe when the measurement is performed, thus, causing defective connection to the package.
FIG. 4 shows an example of the layout of the surface acoustic wave apparatus having the structure shown in FIG. 3 on a piezoelectric substrate. In FIG. 4, squares having a diagonal grid represent bump-bonding electrode pads or wire-bonding electrode pads 105 and squares having diagonal lines represent characteristic selection pads 106 for measuring the filter characteristics in a wafer form using the probe.
However, with the structure of FIG. 3, the insertion loss within the pass band increases as compared with the structure without the surface acoustic wave resonator 102 connected in series. An examination reveals that the insertion loss within the pass band is adversely affected by the parasitic capacitance between the longitudinally-coupled-resonator surface acoustic wave filter 101 and the surface acoustic wave resonator 102.
Particularly, in the structure in FIG. 4 in which the surface acoustic wave resonator 102 is connected in series to at least two IDTs of the longitudinally-coupled-resonator surface acoustic wave filter 101, two signal lines 103 and 104 that connect the longitudinally-coupled-resonator surface acoustic wave filter 101 to the surface acoustic wave resonator 102 surround the electrode pads 105 and the characteristic selection pad 106. Accordingly, large parasitic capacitance is generated between the two signal lines 103 and 104, and the electrode pads 105 and the characteristic selection pad 106 which causes the insertion loss within the pass band to increase.